In regard to hair dyeing agents, a technology of dyeing hair by subjecting oxidative dye intermediates (precursor and coupler) to an oxidative coupling using an oxidizing agent in the coexistence of an alkali agent, is most commonly used. However, oxidizing agents are prone to cause hair damages, and as a result, damages or weakening of the hair, such as a lift-up or peel-off of cuticles, production of cysteic acid, and reduction of lipids in the hair will occur. These damages or weakening is conspicuous in naturally fine and soft hair, and there is known a problem that when such hair is dyed, the hair lacks bounce, body and volume, and therefore, styling of the hair becomes difficult.
Thus, there have been suggested the following methods which achieve a balance between the dyeing properties and impartation of bounce, body and volume in damaged hair, fragile and weakened hair, or naturally fine and soft hair.
(1) A method of adsorbing a film-forming resin and the like, contained in a hair dye, to the hair surface (Patent Document 1).
(2) A method of using a protein hydrolysate and an amino acid in combination (Patent Document 2).
(3) A method of adding a water-soluble substance to the hair, and forming a substance that is insoluble or poorly soluble in water in the hair (Patent Document 3).
Patent Document 1: JP-A-2000-229820
Patent Document 2: JP-A-H07-330559
Patent Document 3: JP-A-H04-338316